electro-stimulator

Muscular Electrostimulation

BackGround

The application of an electrical current escapes from depolarizing the membrane of the muscular or nervous fiber and artificially produces its excitation.

Figure 1: Principle of selective stimulation of the denervated musculature 

It is a technique that involves applying an electric shock directed at a particular muscle, to produce a controlled contraction. It is a therapeutic means, since it controls the work of the muscular fibers with an aim: to repair damaged tissues, to avoid degenerations and / or to avoid traumatisms.

Electrotherapy, employing low-volt, low-frequency impulse currents, has become an accepted practice in the physiotherapy departments. The biological reactions produced by low-volt currents have resulted in the adoption of this therapy in the management of many diseases affecting muscles and nerves. The technique is used for the treatment of paralysis with totally or partially degenerated muscles, for the treatment of pain, muscular spasm and peripheral circulatory disturbances, and for several other applications. Different types of waveforms are used for carrying out specific treatments.The most commonly used pulse waveforms are discussed below. 

Figure 2: Different types of waveforms

Galvanic Current: When a steady flow of direct current is passed through a tissue, its effect is primarily chemical. It causes the movement of ions and their collection at the skin areas lying immediately beneath the electrodes. The effect is manifested most clearly in a bright red coloration which is an expression of hyperaemia (increased blood flow). Galvanic current is also called direct current, galvanism, continuous current or constant current.

Faradic Current: Faradic current is a sequence of pulses with a defined shape and current intensity. The pulse duration is about 1 minute with a triangular waveform and an interval duration of about 20 minutes. Faradic current acts upon muscle tissue and upon the motor nerves to produce muscle contractions. There is no ion transfer and consequently, no chemical effect. This may be used for the treatment of muscle weakness after lengthy immobilization and of disuse atrophy.

Surging Current: If the peak current intensity applied to the patient increases and decreases rhythmically, and the rate of increase and decrease of the peak amplitude is slow, the resulting shape of the current waveform is called a surging current. The main field of application of the Faradic surge current is in the treatment of functional paralysis. The surge rate is usually from 6-60 surges per minute in most of the instruments. The ratio of interval to the duration of the surging is also adjustable so that graded exercise may be administered. This type of current is usually required for the treatment of spasm and pain.

Exponentially Progressive Current: This current is useful for the treatment of severe paralysis. The main advantage of this method lies in the possibility of providing selective stimulation (Fig. 1) for the treatment of the paralysed muscles. This means that the surrounding healthy tissues even in the immediate neighbourhood of the diseased muscles are not stimulated. The slope of the exponential pulse is kept variable.

Analgesic currents stimulation sensitive afferent nerve fibers. Excitomotor currents stimulation of motor efferent fibers.

The type of frequency selected is the key to indicate to the body what type of fiber to activate essentially:

Table 1: Frequency types and their effects

TENS (Transcutaneous Electrical Nerve Stimulation)

Figure 3: Analgesic electrotherapy application

Depolarization of the peripheral sensory nerves to cause, fundamentally, pain reduction.Electrotherapy by TENS modality presents different modalities such as:

• Conventional or high frequency
• Low frequency
• Low frequency in trains or gusts
• Brief or intense

High frequency: Place the electrodes directly on the area painful or also on nervous path, trigger points, acupuncture points, dermatome, contralateral limb. Tickling sensation.

Low frequency: On a muscle of the myotome belonging to the same metamera as the origin of the pain or at the motor point of the muscle. Visible muscular contraction (without movement).

Precautions:

• Proper preservation of electrodes.

• Wet and well rinsed pads

• Good electro-leather contact

• Good skin condition (erosions and wounds)

Contraindications TENS:

• Pacemakers, stimulators (Parkinson's, incontinence)

• Anesthesia or hypoesthesia of the skin.

• Lack of patient cooperation (elderly, children)

• Neoplasms.

• Infections and acute inflammatory processes.

• Precordial zone, carotid sinus, larynx.

• Pregnancy (relative contraindication).

• Gynecological pathologies (metrorrhagia), in applications ABS.

• Epilepsy (relative contraindication).

• Osteosynthesis and metallic stents (non-compensated pulses).

Desing

The design of the electro stimulator is composed of a LM555 timer which will allow us to generate square pulses at different frequencies by the capacitor located in the threshold and potentiometer located in the Discharge, using the equation

And so with the help of this formula and putting a capacitor of 100µF and an R1 100Ω is calculated The value Of the potentiometer to obtain frequencies between 6Hz and 100Hz, and thus it was obtained that the potentiometer should be of 1kΩ getting

Already obtained the frequencies we proceed to manipulate the intensity of operation, this was made daring of a optocoupler, which in turn allowed to make protection to the patient because it performs an isolation of lands, to regulate the intensity It is made by a potentiometer located in the collector of the phototransistor that composes to the optocoupler, for the isolation is done locating the negative part of the diode towards the ground of the circuit and the transmitter of the phototransistor to another Earth, obtaining So the following convention.

Frequency	CORREINTE
6Hz	0.05 mA
100Hz	27th

Results 

In figure four and five, the output signal generated by the circuit implemented with a 555 circuit can be observed. The output frequencies are between 6 and 104 Hertz. To increase the amplitude of the generated signal, an optoacolator powered with 15 volts is used. In this way, a variable current between 0.05 and 0.27 mA is generated. When the signal is amplified with the optocoupler, a small frizzing occurs in the negative part of the signal, as shown in figure 6.

Figure 4:  Signal out of 555.

Figure 5: Signal out of 555.

Figure 6: Signal out optocoupler. ,

Because there was a bad coupling of impedances, when the output signal of the optocoupler was induced to the patient the signal deformed a little, the signal went from being measured to becoming a triangular signal, although this change only occurred at high frequencies, this is due to the impedance of the patient that responds to the change in the frequency of the induced signal. As a solution for the coupling of impedances between the generator circuit of the signal Tens and the patient, a reducer transformer can be used, in addition, to generate an optimal impedance coupling, a circuit of protection to the patient is also being generated. In the implanted circuit an optocoupler was used for the protection of the patient, which is an optimal circuit to generate the protection of the patient, but it is not optimal to generate the coupling of impedances between the patient and the circuit. As the signal is deforming, it was losing its pain treatment properties, so when it reached high frequencies the patient felt itching or tingling.

Figure 7:Figure 8: Output signal with pateint.

Figure 8: Output signal with pateint.

Figure 9: Figure 8: Output signal with pateint.

Conclusions

The signal generated, which is a continuous square wave for treatment of pain through TENS electro-stimulation, is deformed when the circuit is connected to the patient and frequencies greater than 50 Hz are configured, this deformation is possibly generated by the impedance of the patient, for To solve this type of problems, a coupling of impedances between the circuit and the patient must be carried out, for this a transformer can be used, such that the patient is totally isolated from the circuit.

Bibliography

• Webster, J. G.(2006). Encyclodedia of Medical devices and instrumentation. John Wiley & sons, Inc.Khandpur, R.S. 2014. Handbook Of Biomedical Instrumentation, Third Edition. India. McGraw Hill Education.
• Barbara H. Estridge, Anna P. Reynolds, Norma J. Walters (2000). Basic Medical Laboratory Techniques. Cengage Learning.